1. Field of the Invention
The invention generally relates to the field of communications. More specifically the invention relates to interference suppression for use in coded signal communications, such as Code Division Multiple Access (“CDMA”) communications, Wideband CDMA (“WCDMA”) communications, Global Positioning System (“GPS”) communications, Broadband CDMA communications, Universal Mobile Telephone Service (“UMTS”) communications and combinations thereof.
2. Discussion of the Related Art
In CDMA telephony, coded signals are used to communicate between devices. Some typical CDMA telephony systems use combinations of “spreading codes” and “covering codes” to encode signals. These encoded digital signals are used to convey voice, data and/or other forms of digital communication. As used herein spreading codes are pseudorandom number, or pseudo noise, (“PN”) sequence codes and are known to those skilled in the art. As used herein covering codes are orthogonal and/or pseudo-orthogonal codes and are also known those skilled in the art.
A spreading code encodes a data signal by applying a noise-like code sequence to the data at a rate faster than that of the data. Namely, the spreading code is applied to the data such that there are multiple “chips” of the code for any given element of data. Such an application of the spreading code is commonly referred to as direct sequence spreading of the data. A short code is an example of a spreading code. Chips and their associated chip rates are known to those skilled in the art.
A covering code further encodes the signal to provide “channelization” for a signal. For example, each unique covering code as it is applied to a spread signal provides a unique communication channel for the spread signal. This channelization allows a signal to be divided into a number of individual communication channels that may be either shared or assigned to specific users. Covering codes typically include families of codes that are either orthogonal (e.g., Walsh codes) or substantially orthogonal (e.g. Quasi Orthogonal Function, or “QOF” codes).
Interference degrades signal detection and processing capabilities of a receiver by hindering the recovery of a selected signal. While the above-mentioned codes can be used to differentiate signals, interference from unwanted signals is a persistent problem in CDMA communications. For example, interference can be the result of receiving one or more unwanted signals simultaneously with a selected signal. These unwanted signals can be coded signals having properties that are similar to that of the selected signal. Because of code similarities and signal energy, the coded signals often have a tendency to interfere with one another and disrupt the reception of the selected signal. The lack of orthogonality of the received signals results in “leakage” from one signal into another. Examples of such interference include “cross-channel” interference and “co-channel” interference.
Co-channel interference may include multipath interference from the same transmitter, wherein a transmitted signal takes unique paths that causes one path (e.g., an interfering signal path) and another path (e.g., a selected signal path) to differentially arrive at a receiver, thereby hindering reception of the selected path. Cross-channel interference may include interference caused by signal paths of other transmitters hindering the reception of the selected signal path. Such interference can corrupt data as long as it is present in any substantial form.
“Rake” receivers operate in multipath environments that include such interference to improve reception of the selected signal via the combination of signal paths. Rake receivers have a plurality of “fingers,” wherein each finger of the rake receiver independently estimates channel gain and other signal characteristics, such as phase, of the selected signal path to more accurately demodulate and subsequently retrieve underlying data of the selected signal by combining multiple copies of the signal. Although rake receivers can improve data retrieval of a received signal, present rake receivers do not reduce cross-channel interference and/or co-channel interference.